Background: Ovarian carcinogenesis is probably in large part due to factors associated with reproduction. During pregnancy, large amounts of estrogens and progesterones are produced by the placenta. The levels of pregnancy hormones are influenced by pregnancy characteristics, such as gestational age. Despite the evidence that risk of ovarian cancer is reduced with increasing parity, while incomplete pregnancies, such as spontaneous abortions, appear not to influence risk, studies of associations between indicators of hormonal exposures during pregnancy and subsequent risks of ovarian cancer are lacking. Specific aims: 1. To study risk of ovarian cancer by markers of hormone exposures during pregnancy, including birthweight, gestational age, single or multiple birth, pregnancy-induced hypertensive diseases, and placental weight; 2. to study if the protective effects of increasing parity and a high age at first birth are influenced by markers of hormone exposures during pregnancy; and 3. to assess the importance of other factors on ovarian cancer risk. Methods: We will establish a cohort of more than 1.2 million women who delivered their first infant between 1973 and 2000, and were included in the nation-wide Swedish Medical Birth Register. We plan to retrieve information about maternal characteristics, pregnancy complications, placental weight, birth characteristics for all births to these women, and women will be followed up in population-based registers with regards to gynecological surgery, vital status, and ovarian cancer. We estimate to include at least 1,000 women with ovarian cancer. We will use Cox proportional hazards models and Poisson regression models to estimate ovarian cancer incidence rate ratios for exposures of interest. Significance: Little new knowledge of the etiology of ovarian cancer has been gained during recent decades. The Swedish research registers will allow us to perform a large cost-effective cohort study where information on markers of hormone exposure during pregnancy has been collected prospectively.